If you insist on staying with him -- you must also be prepared to serve as a Source of Narcissistic Supply, an alternative to the supply provided by his former lover. You must brace yourself: serving as a Narcissistic Supply Source is an onerous task, a full time job and a very ungrateful one at that. The narcissist's thirst for adulation, admiration, worship, approval, and attention can never by quenched. It is a Sisyphean, mind-numbing effort, which heralds only additional demands and disgruntled, critical, humiliating tirades by the narcissist.
That you are afraid to confront reality is normal. You are afraid to set clear alternatives. You are afraid that he will leave you. You are afraid that he will prefer her to you. AND YOU MAY WELL BE RIGHT. But if this is the case and you go on living with him and tormenting yourself -- it is unhealthy.
If you have find it difficult to confront the fact that it is all over between you, that your relationship is an empty shell, that your husband is with another woman -- do not hesitate to seek help from professionals and non-professionals alike. But do not let this situation fester into psychological gangrene. Amputate now while you can.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)